As described in the specification 3GPP TS 23.272, the Circuit Switched domain Fallback feature (CSFB) in the Evolved Packet System (EPS) allows for provisioning of voice and other CS-domain services (e.g. SMS) by reuse of the CS infrastructure when the user equipment (UE) is served by Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
Specifically for the case of Mobile Terminated (MT) voice calls, the CSFB feature allows to page the mobile for CS domain MT calls while the mobile camps in Idle mode on E-UTRAN cell or while the mobile is connected to the EPC/E-UTRAN.
In both cases the mobile is eventually redirected to the legacy GERAN/UTRAN (GSM EDGE Radio Access Network/UMTS Terrestrial Radio Access Network) access over which the CS domain call is eventually established.
In the pre-CS fallback terminating call establishment cases, the call management signalling defined in the 3GPP 24.008 Technical Specification triggers state transitions in the network, as the call establishment progresses. In particular, once a proper radio channel has been allocated to the called UE, the called UE may generate an ALERTING message, which indicates the called user is being alerted, and triggers the network to generate a ring-back tone towards the calling party.
However, in CS Fallback context, when the called user was in idle mode on the E-UTRAN side, if the network waits for the generation by the called UE of the ALERTING message (which can only be sent after the CS Fallback procedure has taken place), the calling user will have to wait for a very long time before being informed about the fact that the network connected with the called user.
In CS Fallback context, when the called user was in connected mode on the E-UTRAN side, the called user was already alerted of the MT call and requested to accept/refuse the incoming call while it was still attached to E-UTRAN.
If the called UE sent an ALERTING message to the network after getting to UTRAN/GERAN, a ring-back tone would be generated towards the calling UE but that would mean the called user would need to accept the call a second time or to wait for some additional time for the call to be established (if waiting for some time for the calling user to hear a ringback tone before connecting the users). If the called UE sent a CALL CONFIRMED message directly followed by a CONNECT message without previous ALERTING message, the calling user will not have heard a ring-back tone before hearing the called user.
Overall this will result in poor user experience.
Embodiments of the present invention will improve the situation. In particular, embodiments of the invention aim at improving the user experience of the calling party in the case of CS domain Fallback with mobile terminated calls.